Bacteria use the electrochemical energy generated at the cell membrane to power flagella rotation and are thus motile. Furthermore, they can sense changes in nutrients in their environment and respond by changing the frequency of reversal of flagella rotation, thus directing their own movement. We have identified specific genes and polypeptides that act in the cell membrane to mediate the transduction of energy into flagellar rotation as well as to integrate sensory signals and to transmit them to the flagellar apparatus. A family of transmembrane proteins was found that has many characteristics in common with the cell surface receptors that has been described in a variety of complex eukaryotic systems. This group of proteins is able to sample the environment by binding specific ligands and to transmit the information to the cytoplasmic side of membrane. The proteins are themselves modified and they appear to act as modulators of the signals that they transmit. Other membrane associated polypeptides are required for flagella rotation. They are apparently directly involved in a conversion of energy into flagellar motion. We plan to focus our attention on the genes that encode all of these membrane proteins. Genetic and molecular analysis will be applied to determine relationships between the gene, the gene product, and its function. The DNA sequences of the genes that encode some of these membrane proteins will be determined. Site-specific mutagenesis will be used to alter the gene products in well-defined ways. The altered genes will be used to replace the wild-type gene. Studies of the behavior of these mutant strains and the mutant proteins will be used to delineate the functions of specific domains in these genes and in the proteins. By understanding how the gene products function and how they evolved, we will gain insight into the general properties and mechanisms involved in a wide variety of biological processes that require transmembrane signaling and energy transduction. These insights can be applied directly in the bacterial system since motility and chemotaxis are known to be important factors in bacterial pathogenicity. They will also be applicable to other systems where chemoreceptors play essential roles in cell function.